Parts of the Great Wall date from the Warring States Period, when each state built  a fortified wall along its border  to ward  off 
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possible  invasions from  its  hostile  neighbors. After emperor Qin Shi-huang  succeeded  in unifying China, he reinforced  the walls 
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along the northern borders of the original states and had them connected into one defensive system traversing from Lintao in Gansu 
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in the west to Liaodong in the east. Reinforcement  and renovations  were  constantly carried out  during successive dynasties. The 
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present shape of the Great Wall owes much to the 100-year-old large-scale expansion and  rebuilding during the Ming Dynasty.